Voltage converter for converting an input voltage to an output voltage and driving circuit comprising a voltage converter

ABSTRACT

The invention regards an improved voltage converter with increased current capability. The voltage converter architecture may be configured by software. In the prior art programmable charge pumps have been configured in such a way, that the unused stages were simply short circuited by a decoding logic. According to the invention these stages are used to increase the current capability of the first pumping stage. In particular the result is an increase in current capability of a proposed charge pump device by 10% to 15% without the need of additional parts and within the same area.

The invention regards a voltage converter for converting an inputvoltage to an output voltage comprising a plurality of cascaded voltagemultipliers and control circuitry for controlling the plurality ofvoltage multipliers. Further, the invention leads to a driving circuitcomprising such a voltage converter.

A voltage converter, such as a charge pump device or other devicecomprising voltage multipliers, is used to generate a higher voltagethan a supply voltage available for an application. A voltage convertermay have several voltage multipliers arranged in stages in a kind ofcascade. A charge storage element of the first stage may be charged upona switching event of a driver driving the first stage. A furtherswitching element of the first stage may be in an opened position inthis case. Thereafter the further switching element may be closed sothat the charge may be supplied to the next stage. The charge storageelement of the subsequent stage may then be charged upon a switchingevent of a driver driving the subsequent stage. Thus, a charge stored inthe first stage is forwarded to one or more subsequent stages, where itis added to the charge of such a subsequent stage, so that a highervoltage is generated and can be provided to a device of an application.

An advantage of using charge pumps is that generally no additionalbypass switches are needed and a voltage converter of such kind can thusbe relatively simply construed. A charge pump may be composed of acascade of several stages, whereby each stage contains at least oneswitch or diode, a charge-storing element, usually realized as acapacitor, and a driver. The driver commands the charge storing elementsand may be operated by periodical signals or phases.

A voltage converter is known from a publication in the IEEE Journal ofsolid-state circuits, vol. SC-11, No. 3, June 1976, pages 374-378. Inthe publication the voltage converter output voltage serves to provide ahigh supply voltage in a NMOS integrated circuit application. Further, avoltage converter may be used in a liquid crystal display (LCD) driveras an application to generate a bias voltage required by a drivingscheme. Yet another application is the generation of a high voltagenecessary to write a flash memory.

Basic architectures of voltage converters employed in LCD-driving IC'smay be programmable with regard to the application to which the voltageis supplied. Such a voltage converter as that mentioned in theintroduction allows an appropriate number of selected voltagemultipliers to be activated, i.e., if a relatively low output voltage isdesired, the number of active voltage multipliers is relatively low,whereas if a relatively high output voltage is desired, the number ofactive voltage multipliers is relatively high. Thus, an unnecessarilyhigh number of active voltage multipliers can be avoided, therebyavoiding a low power efficiency of the voltage converter. Thenon-active, i.e. disabled voltage multipliers should be bypassed bymeans of switches or shunted parallel to an active voltage multiplier.Some types of voltage multiplier allow a switch within the voltagemultiplier to be used, in order to bypass the respective voltagemultiplier if it is in a non-active operating mode.

LCD modules find a large application in cellular phones and otherhand-held tools such as organizers, laptops, PDAs etc. An availablesupply voltage for an analogue block may be 2.8 V and an LCD graphicdisplay may be driven with voltages of 6 V to 16 V and should also beoperable within a large range of supply voltages. A range of outputvoltages is desired for different kinds of applications.

However, in the LCD driving integrated circuits (IC's) the trendnowadays is towards developing ever increasing resolutions and colordisplays, in particular for mobile terminals such as phones, PDA's etc.These displays need a large current at a voltage approaching 20 V, whichhas to be generated by the LCD-driving IC. In the actual LCD-drivingIC's the amount of charging capability of a charge storage element,particularly a capacitor value, is limited and the amount of necessarycurrents is increasing. The achievable voltage and required current aresubject to compromise, but still the need for an increased currentcapability is generally becoming more acute. In particular, anincreasingly large current is usually drawn from a smaller capacitor andthe voltage drop due to a charge transfer is increasingly large. Thislarge voltage drop may be multiplied by all the stages of a voltageconverter, thus degrading the performance.

This is where the invention comes in, the object of which is to proposean apparatus and a method of converting an input voltage to an outputvoltage, whereby an improved current capability should be achieved.

As regards the apparatus the object is solved by a voltage converteraccording to claim 1, which claims a voltage converter for converting aninput voltage to an output voltage comprising a plurality of cascadedvoltage multipliers and control circuitry for controlling the pluralityof voltage multipliers, wherein in accordance with the invention thecontrol circuitry comprises a switching means for activating at leastone first voltage multiplier selected from the plurality of voltagemultipliers and for switching at least one further voltage multiplierlocated in the cascade before the first voltage multiplier in the sameway as the first voltage multiplier.

Further, as regards the apparatus the object is solved by a drivingcircuit according to claim 14, which is a driving circuit, comprising avoltage converter at present, in particular a driving circuit for adisplay device.

As regards the method the object is solved by a method of converting aninput voltage to an output voltage by means of a voltage convertercomprising a plurality of cascaded voltage multipliers, wherein inaccordance with the invention at least one first voltage multiplierselected from the plurality of voltage multipliers is activated and atleast one further voltage multiplier located in the cascade before thefirst voltage multiplier is switched in the same way as the firstvoltage multiplier.

The proposed invention has arisen from the desire to further increasethe current capability of a voltage converter or a driving circuit, inparticular when not all stages of a voltage converter are used formultiplying a voltage. The invention has realized that conventionallythe unused multipliers and/or charge storage elements of a multipliermerely play a passive role. They merely appear as decoupling orbuffering elements. According to the inventive idea now these are usedto increase the strength of a first voltage multiplier selected from theplurality of voltage multipliers, when switched as proposed by aswitching means. In particular when using a charge pump as a voltagemultiplier, if not all stages of the charge pump are used for a pumpingcharge, the unused capacitors no longer play a passive role indecoupling or buffering capacitors, but are used to increase thestrength of the first pumping stage together with their bottom platedriver.

As a main advantage of the invention the first and further voltagemultiplier may be switched in the same way by changing the logiccircuitry only. Advantageously no increase in area or number of analogueparts is foreseen. Depending on the number of stages and the supplyvoltage an increase in current capability of 10% to 15% may thus beachieved.

Further, developed configurations of the invention are outlined in thedependent claims.

Switching comprises activating and disabling. To activate the respectivestage is preferably switched to the supply voltage V_(dd) potential. Themultiplier is thus in an active mode. To disable the respective stagemay be switched to ground voltage. The multiplier is thus in a passivemode.

Advantageously the first voltage multiplier is one of a number ofactivated voltage multipliers also located in the cascade at a second orhigher order stage at most, in particular located in a sequence ofstages at the end of the cascade.

Advantageously the further voltage multiplier is one of a number offurther voltage multipliers also located at the first or higher orderstages of the cascade, in particular located in a sequence of stages atthe beginning of the cascade.

At least one of a plurality of voltage multipliers is preferably formedby a charge pump.

Such a voltage pump comprises a charge storage element, in particular acapacitor, a switch, in particular a MOSFET switch, and a driver, inparticular a bottom plate driver. The charge storage element may beon-chip or off-chip.

Also, one or more of the voltage multipliers may have at least one clockinput. The control circuitry is preferably connected to a clock inputfor supplying a clock signal to a voltage multiplier for controlling thevoltage multiplier.

In a preferred configuration the voltage converter is programmable. Inparticular the switching means is a programmable logic device. Such alogic device is advantageously driven by a programming means foroperating the switching means as a function of the output and/or inputvoltage, i.e. depending on the application. Thus, the multiplicationfactor of a voltage converter is advantageously adapted to the actualapplication. In particular, not all stages of the converter may be usedto generate an output voltage below the maximal possible voltage, whenall stages of the converter are to be used.

In particular, the programming means comprises a software code sectioncapable of activating one or more first voltage multipliers in the caseof an input voltage, which is insufficient for the actual application,i.e. the respective multipliers are held in an active operating mode.Further, the programming means may comprise a software code section fordisabling a number of voltage multipliers selected from the plurality ofvoltage multipliers in the case of an input voltage sufficient for theactual application, i.e. the respective multipliers are held in adisabled or passive operating mode. In particular for this specificsituation the programming means preferably comprises a software codesection capable of selecting one or more further voltage multipliersfrom the disabled voltage multipliers to be switched in the same way asthe first voltage multiplier.

Advantageously a re-configurable voltage converter constitutes one ofthe above measures or a combination thereof.

A voltage converter and/or a driving circuit as proposed worksadvantageously under a current load of 0.1 mA to 10 mA.

Preferred embodiments of the invention will now be described in adetailed description with reference to the accompanying drawings. Theseare meant to show examples to clarify the proposed concept in connectionwith the detailed description of the preferred embodiment and incomparison to prior art. While the considered preferred embodiment willbe illustrated and described, it should of course be understood thatvarious modifications and changes in form or detail could readily bemade without departing from the spirit of the invention. It is thereforeintended that the invention may not be limited to the exact form anddetail shown and described herein nor to anything less than the whole ofthe invention disclosed herein and as claimed hereinafter. Further thefeatures described in the description, the drawings and the claimsdisclosing the invention may be essential for the invention, consideredalone or in combination.

The drawing shows in:

FIG. 1: a schematic charge pump architecture;

FIG. 2 a: a schematic view of a prior art charge pump-switching scheme;

FIG. 2 b: a charge pump-switching scheme according to the preferredembodiment;

FIG. 2 c: an equivalent circuit of the proposed embodiment of FIG. 2 b;

FIG. 3: a preferred display device embodiment comprising a drivingcircuit containing a preferred charge pump.

The preferred voltage converter embodiment is described with regard to aprogrammable charge pump as shown in FIG. 1. A programmable charge pump1 has a series of stages S₁ . . . S_(N), S_(N+1) . . . S_(max) eachcontaining a capacitor C_(stage) as a charge storage element, a MOSFETas a switch Sw₁ . . . Sw_(N), Sw_(N+1) . . . S_(max) and a bottom platedriver as a switching means (not shown). Each respective charge pumps S₁. . . S_(N), S_(N+1) . . . S_(max) further comprises a buffer B₁ . . .BF_(N), BF_(N+1) . . . BF_(max) with an input I₁ . . . I_(N), I_(N+1) .. . I_(max), e.g. for receiving a clock input signal and a voltageV_(dd) as indicated in the Figure. An input voltage V_(dd) is generatedby a voltage source between the input terminal IP and the groundterminal GND and is supplied to the charge pump device 1. The stages areconnected in a cascade, one after each other. With every stage one inputsupply voltage V_(dd) may be gained. This is true for the idealsituation of no output current. The clock may also be generatedadaptively with several phases. The above-outlined ideal situation hasto be corrected as there are internal losses, these losses being mainlydue to dissipated currents by switch resistances when in an“on”-position and one or more bottom plate drivers. A single or aplurality of bottom plate drivers are capable of actuating each of thecharge pumps S₁ . . . S_(N), S_(N+1) . . . S_(max) and in particular theswitches Sw₁ . . . Sw_(N), Sw_(N+1) . . . Sw_(max). A driver is operablewhen connected to each of the charge pumps S₁ . . . S_(N), S_(N+1) . . .S_(max) in a suitable way, e.g. by operating the input terminals IP, I₁. . . I_(N), I_(N+1) . . . I_(max) and the switches Sw₁ . . . Sw_(N),Sw_(N+1) . . . Sw_(max). A state in which a charge pump may be operatedis indicated in FIGS. “1” and “2” respectively. In the output stage witha capacitor C_(buffer) the output voltage V_(out) is generated betweenthe output terminal IO and the ground terminal GND.

The stages S₁ . . . S_(N), S_(N+1) . . . Sw_(max) of FIG. 1 and thesizing thereof are identical to each other in their construction.However, this is not the optimal case and has been chosen only toachieve a simple design with the advantage of a short design time.Further developed voltage converter embodiments may preferably bedesigned in a different way. In particular, stages S₁, S₂ . . . S_(N)close to the voltage booster input may have a stronger layout thanS_(N), S_(N+1) . . . S_(max) closer to the output.

The improvement of the preferred charge pump embodiment with regard toconventional charge pumps is explained in the following.

In a single clock cycle the charge is usually transferred from thecapacitor of a stage N into the capacitor of the following stage N+1. Atthe beginning of the clock cycle, the capacitor of the stage N+1 is notdischarged completely. In fact, not all of its charge Q acquired in thepreceding cycle is usually transferred to the next stage N+2 within onecycle. In general, the quantity Q=I_(load)/2f is transferred, where f isthe clock frequency. This corresponds to a voltage difference ofΔU=Q/C=I/2fC. If the capacitor has a large capacitance C for the sameI_(load) the voltage drop ΔU due to this current is smaller than with acapacitor of small capacitance, which can be drawn from the latterformula. However, in the actual LCD-driving IC's the capacitor value islimited and therefore the amount of currents is becoming increasinglylarge. Usually a current is in the order of 1 mA and higher. Such avoltage drop has to be assumed for each stage 1 . . . N. Therefore, theassumption of an efficiency of 100% I_(load)=I_(supply)/N also holds. Asa result the voltage and current achieved are subject to compromises,which have to be made for each application.

As the first stage of the charge pump experiences the following stagesas a load, this means that the need of a big current capability becomesmore and more acute when approaching the beginning stage S₁ of the chainof stages S₁ . . . S_(N), S_(N+1) . . . S_(max). In particular, if alarge current is drawn from a small capacitor C_(stage), then thevoltage drop ΔU due to the charge transfer is large as well. Asoutlined, this charge voltage drop is multiplied by the following stagesN−1 times, thus degrading the performance of a conventional charge pump,as an example of a conventional voltage converter.

To remove such a problem the multiplication factor is programmed asoutlined in the following:

Assuming V_(ott)=N_(max)*V_(dd) one may choose to generate onlyV_(out)=N*V_(dd) by setting the (N_(max)−N) switches Sw to theconducting state and, correspondingly, the bottom plate driverspermanently to ground GND. In this case the first active charge pump Aof FIG. 2 a is not the first charge pump in the cascade, but may be anycharge pump in the cascade, e.g. in stage S_(N+1). This means that therespective charge pumps located before the active one are disabled, asis indicated in FIG. 1 by “0”. Thus (N_(max)−N) stages are disabled.Within the function of the whole device the disabled stages usuallymerely play a filtering element role for the input supply voltage V_(dd)or as a charge tank permanently charged by the input supply voltageV_(dd). This in fact has a certain benefit with regard to theperformance of the charge pump device. However, it also has a drawback,the disabled stages are used only as a decoupling element or a “chargetank” in front of the pumping stages and therefore only play a passiverole.

According to the preferred embodiment shown in FIG. 2 b use is made ofthese disabled stages and in fact results in an increase of the currentcapability of a charge pump device 1. To put it simply, the preferredembodiment introduces a means of more effectively using the “disabled”stages.

As an example the following describes a way of reinforcing the firstactive pumping stage A shown in FIG. 2 b, in order to achieve thedesired output voltage in less time and increase the current provided bythe charge pump.

The reinforcement of first pumping stage A is only described with regardto FIGS. 2 b and 2 c to illustrate the basic principle of effectively“using” the “disabled” stages, e.g. stages S₁ . . . S_(N), and may begeneralized according to the demands of an actual application.

FIG. 2 a illustrates the situation in which a first stage is forced toground voltage by the bottom plate driver.

The preferred embodiment 1b is illustrated in FIG. 2 b. A logic deviceis giving the bottom plate driver a command to switch the first stage1^(st) between V_(dd) (activated) and ground (disabled) in the same wayas for the first active pumping stage A. The first active pumping stageA in FIG. 2 b is in fact the second stage or a higher stage S_(N+1)located in the cascade at most. The circuit 1 b of FIG. 2 b isequivalent to 1 c in FIG. 2 c, from which it is immediately clear thatthe input stage A=A′ now has a capacitance of 2*C_(stage) and theequivalent resistance of A′ is R_(d)/2 because the two bottom platedrivers are connected in parallel. Consequently, the charge stored inthe second stage A=A′ at the end of the charge transfer is higher thanin a conventional charge pump device circuit as shown in FIG. 2 a.

The attached graphs Ga and Gb in FIGS. 2 a and 2 b correspond tocircuits 1 a and 1 b in FIG. 2 a and FIG. 2 b respectively. From thegraphs it is clear that in the second stage A the charge transferredinto the capacitor will be passed forward to stage B in the preferredembodiment 1 b of FIG. 2 b is Q=I_(load)/3f instead of Q=I_(load)/4f ina conventional circuit 1 a of FIG. 2 a.

It should of course be understood that if two stages of the kindindicated by 1^(st) in FIG. 2 b are available and can be disabled andassuming that both were added to the first active stage A as outlinedabove, then the charge to be transferred in one cycle would beQ=3I_(load)/8f. Any number of available stages, which can be disabled,can be added to the first active stage A according to the preferredembodiment.

Clearly, when the output capacity C_(buffer) of the charge pump isheavily loaded, the present embodiment helps to alleviate the effort.The improvement has been examined in several experiments. In particular,the effect of the improvement has been simulated with a charge pumpdevice with five stages, i.e. 6 charge pumps, for which the 1^(st) stagehas merely been used as outlined in FIG. 2 b to increase the equivalentcapacitance of stage two or a higher stage in the cascade. The result ofthe simulation proved that a preferred embodiment 1 b is able to achievethe same voltage within seven clock cycles, as compared to eight clockcycles needed by a conventional device 1 a of FIG. 2 a. One cycle lessis needed by a preferred embodiment. If one considers the load currentI_(load) required in order to achieve the same voltage with the priorart approach of FIG. 1 a and the outlined approach of the preferredembodiment of FIGS. 2 b and 2 c of merely improving the first stage inthe cascade, the improvement amounts to 12.5% in the current capability.Consequently the prior art charge pump would give 12.5% less current forthe same voltage. In other words, a conventional device would take 12.5%more time to recover from a large peak of current absorbed by the load.

This improvement is even more effective considering the fact that theabove example has been outlined without taking any load current and anylosses for achieving (N_(stages)+1)*V_(dd) into consideration. However,as there is usually a load current in charging, the voltage generallyachieved is lower. To obtain a voltage under load conditions, usuallythe pump has to be over-dimensioned by an increased number of stages inthe cascade. Therefore, the input voltage range usually has to be largerthan in theory. In order to achieve the target output voltage V_(out)most efficiently the number of active stages can be programmed toaddress a variety of applications. If there is a sufficient inputvoltage V_(dd), one or two stages may be disabled by software. Byoperating those stages in such a way that they increase the value of thefirst capacitor one is also able to benefit from the increased currentcapability. In fact, this is achieved without any area being wasted, asthe proposed concept may more or less be achieved by merely changing thedecoder logic for the bottom plate drivers. Such a decoder is of lowvoltage and small compared to other devices. Usually the decodercomprises only several logic gates.

To realize the proposed scheme changes have to be made only for some fewitems of a charge pump device 1. The logic which controls the bottomplate drivers of the stages S₁ . . . S_(N), S_(N+1) . . . S_(max) in thecascade has to be modified as proposed with the preferred embodiment ofFIG. 2 b. As a consequence of the proposed charge pump device thevoltage on the second pumping stage capacitor A in the preferredembodiment differs from the one of a conventional device as outlined inFIGS. 2 a and 2 b. In fact, under the outlined conditions of load V_(dd)and target voltage V_(out), the proposed concept is the only one toobtain a current efficiency increase in the amount shown above.

In summary, the proposed concept allows to realize a re-configurablecharge pump software architecture. The benefit of this is the increaseof the current capability should the wanted multiplication factor belower than (N_(stages)+1).

The field of application of an improved charge pump may be any driverfor a grayscale or a color display. As outlined in FIG. 3 such a display11 may be any display for a LCD or PolyLED technology or any otherapplication needing a voltage booster working under current loads of 0.1mA to 10 mA. The preferred embodiment of a driving circuit 12 may bepart of a display module 1 1, built with an LCD cell 14 and additionallya display driver IC 12 as proposed. The IC 12 is preferably mounted onglass. Also such a display driver IC could be connected (15, 15 a) withTCP or a foil. A charge pump 12 a, as a preferred voltage converterembodiment, is part of the illustrated driver IC 12 of FIG. 3, as apreferred driving circuit embodiment. The charge pump provides the highvoltage necessary to drive the LCD cell 14. The display module may bebuilt, for example, in small portable devices 10 as cellular phones andPersonal Digital Assistants (PDAs) as shown in FIG. 3.

1. A voltage converter for converting an input voltage to an output voltage comprising a plurality of cascaded voltage multipliers and control circuitry for controlling the plurality of voltage multipliers, characterized in that the control circuitry comprises a switching means for activating at least one first voltage multiplier selected from the plurality of voltage multipliers and for switching at least one further voltage multiplier located in the cascade before the first voltage multiplier in the same way as the first voltage multiplier.
 2. A voltage converter according to claim 1, characterized in that switching comprises activating and/or disabling.
 3. A voltage converter according to claim 1, characterized in that the first voltage multiplier is one of a number of activated voltage multipliers also located in the cascade at a second or higher order stage at most, in particular located in a sequence of stages at the end of the cascade.
 4. A voltage converter according to claim 1, characterized in that the further voltage multiplier is one of a number of further voltage multipliers located at the first or higher order stages of the cascade, in particular located in a sequence of stages at the beginning of the cascade.
 5. A voltage converter according claim 1, characterized in that at least one of the plurality of voltage multipliers is formed by a charge pump.
 6. A voltage converter according to claim 1, characterized in that the charge pump comprises a charge storage element, in particular a capacitor, a switch, in particular a MOSFET switch, and a driver, in particular a bottom plate driver.
 7. A voltage converter according to claim 1, characterized in that one or more of the voltage multipliers have at least one clock input.
 8. A voltage converter as claimed in claim 7, characterized in that the control circuitry is connected to the clock input for supplying a clock signal to the voltage multiplier for controlling the voltage multiplier.
 9. A voltage converter according to claim 1, characterized in that the switching means is a programmable logic device.
 10. A voltage converter as claimed in claim 9, characterized by a programming means for operating the switching means as a function of the output and/or the input voltage.
 11. A voltage converter as claimed in claim 9, characterized in that the programming means comprises a software code section capable of activating a number of one or more first voltage multipliers in case of insufficient input voltage.
 12. A voltage converter as claimed in claim 9, characterized in that the programming means comprises a software code section for disabling a number of voltage multipliers selected from the plurality of voltage multipliers in case of sufficient input voltage.
 13. A voltage converter as claimed in claim 9, characterized in that the programming means comprises a software code section capable of selecting a number of one or more further voltage multipliers from the disabled voltage multipliers for switching the further voltage multipliers in the same way as the activated first voltage multiplier.
 14. Driving circuit, comprising a voltage converter as claimed in claim 1, in particular a driving circuit for a display device.
 15. Driving circuit as claimed in claim 14, working under a current load of 0.1 mA to 10 mA.
 16. Method of converting an input voltage to an output voltage by means of a voltage converter comprising a plurality of cascaded voltage multipliers, characterized in that at least one first voltage multiplier selected from the plurality of voltage multipliers is activated and at least one further voltage multiplier located in the cascade before the first voltage multiplier is switched in the same way as the first voltage multiplier. 